This invention relates to a micromachined capacitive electrical component in general. In particular the invention relates to a capacitive transducer such as a condenser microphone. Such micromachined systems are often referred to as Micro Electro-Mechanical Systems (MEMS). The invention is particularly useful in a condenser microphone that can be used eg with standard sound measurement equipment using a high polarization voltage.
In principle, a condenser microphone comprises a thin diaphragm that is mounted in close proximity to a back plate. The thin diaphragm is constrained at its edges, so that it is able to deflect when sound pressure is acting on it. Together the diaphragm and back plate form an electric capacitor, where the capacitance changes when sound pressure deflects the diaphragm. In use, the capacitor will be charged using a DC voltage, usually called polarization voltage. When the capacitance varies due to a varying sound pressure, an AC voltage that is proportional to the sound pressure will be superimposed on the DC voltage. The AC voltage is used as output signal of the microphone.
The polarization voltage Vpol is applied by an external voltage source via a resistor (see FIG. 1). The resistance of this resistor must be so high that it ensures an essentially constant charge on the microphone, even when the capacitance changes due to sound pressure acting on the diaphragm. The value of this bias resistor is typically 15 Gxcexa9. A high polarization voltage is used in standard scientific and industrial sound measurement equipmentxe2x80x94more than 100 V, and usually 200 V. Using a high polarization voltage dates back to measurement equipment based on vacuum tubes and technological limitations in fabrication of condenser microphones using precision mechanics. Although a lower polarization voltage would be more compatible with electronics of today, using a high polarization voltage has become a standard in sound measurement equipment during the years. Therefore, microphones intended for sound measurement should preferably be designed for use with a polarization voltage up to at least 200 V in order to be compatible with existing measuring equipment.
Micromachined components that are usually developed for use in low-voltage systemsxe2x80x94typically  less than 10 V. In condenser microphone chips, between the diaphragm electrode and the back plate electrode there is an air gap. The typical thickness of the air gap of known micromachined microphone chips is less than 5 xcexcm, whereas a typical microphone for scientific and industrial precision sound measurement has a 20 xcexcm air gap. The difference in air gap thickness is necessitated by the difference in operating voltage. Micromachined microphone chips need a small air gap to obtain a field strength in the air gap that is high enough to get an acceptable sensitivity for a low polarization voltage. However, the electrical field strength cannot be increased without limit. Due to the polarization voltage electrostatic forces attract the diaphragm to the back plate, and above a critical electrical field strength the diaphragm xe2x80x9ccollapsesxe2x80x9d and snaps to the back plate. The collapse voltage Vc is given by the formula       V    c    =                    1.578        ·        σ        ·        t        ·                  D          3                                      ϵ          0                ·                  R          2                    
where "sgr" is the diaphragm stress, t is the diaphragm thickness, D is the air gap thickness, xcex50 is the vacuum permittivity, and R is the diaphragm radius. It can be seen from the formula that for a constant collapse voltage, a reduction of the air gap thickness must be compensated by an increase of the diaphragm stiffness ("sgr"xc2x7t/R2). Consequently, a typical micromachined microphone with an air gap of less than 10 xcexcm needs a diaphragm with a very high stiffness in order to operate at 200 V. For example, a microphone with a diaphragm radius of 0.5 mm and an air gap of 10 xcexcm needs a stiffness of 87.5 N/m, which can be obtained by a 0.5 xcexcm thick diaphragm with a stress of 175 MPa. This is certainly not impossible to manufacture, but the problem is that the high diaphragm stiffness also gives a microphone with a very low sensitivity and consequently a very high noise level. In this example, a noise level of more than 45 dB can be expected, which is too high for most sound measurement applications. In other words, a microphone that should be able to operate using 200 V polarization voltage and at the same time have a low noise level must be provided with an air gap with a thickness of more than 10 xcexcm.
Using an air gap thickness of much more than 20 xcexcm is not recommended either, since then the capacitance of the microphone thereby becomes so small that it becomes difficult to measure the microphone signal, due to the signal attenuation caused by parasitic capacitances in parallel with the microphone.
Another issue concerning the use of 200 V polarization voltage is electrical insulation between the diaphragm electrode and the back plate electrode. To ensure an extremely stable sensitivity, it is critical that the leakage resistance of a sound measurement microphone is highxe2x80x94at least 1000 times the value of the bias resistor. This corresponds to 15 Txcexa9, which value must be maintained even under extreme conditions, such as 200 V polarization voltage in combination with high humidity and temperature.
The known principle of the construction of a microphone chip with an electrically conducting diaphragm is shown in FIG. 2. At the edges of the chip, a conducting diaphragm 1 and back plate 3 provided with holes 5 are attached to a silicon frame 2. At this connection, insulator 4 separates the back plate electrode and the diaphragm electrode. Due to the nature of thin-film deposition processes, the thickness of the insulator 4 is limited to values of the order of 1-3 xcexcm. The leakage resistance of the microphone chip is determined by the quality of the insulator 4.
Silicon microphone chips can also be made using insulating diaphragm materials. Such known constructions are shown in FIG. 3 and FIG. 4. The diaphragm of the microphone chip in FIG. 3 is provided with a diaphragm electrode 6. In this case, the insulating diaphragm acts as insulator between the diaphragm electrode and the back plate electrode. It is also possible to provide the insulating diaphragm with an electrode 7 on the side facing the air gap. This design is shown in FIG. 4. A conductive layer on the outside of the diaphragm and chip is still needed to provide effective shielding against electromagnetic interference (EMI).
The leakage resistance of insulating materials in FIGS. 2-4 comprises two components, the bulk resistance and the surface resistance. The surface resistance is determined by the insulator material, by the condition of the surface (cleanliness, humidity, surface treatment and finish) and by the lateral dimensions of the insulator (path length that the leakage current has to travel between the diaphragm electrode and the back plate electrode). The bulk resistance is determined by the insulator material, the thickness of the insulator, and by the electrical field strength in the insulator. At higher field strengths, an insulating material shows a leakage current density J increasing exponentially with the square root of the field strength E, which is typical for the Poole-Frenkel conduction mechanism in insulators (see for information in S. M. Sze, xe2x80x9cPhysics of semiconductor devicesxe2x80x9d, 2nd ed., John Wiley and Sons, New York, 1981, pp. 402-404). The exponential increase in leakage current gives an exponentially decreasing leakage resistance of the microphone. The exact value of the leakage resistance at these high field strengths depends on the material and the thickness (field strength!). When testing the bulk insulating properties of silicon nitride films, we have measured a leakage resistance of more than 10 Txcexa9 at 100 V/xcexcm across the silicon nitride, whereas the resistance decreased to 1 Gxcexa9 at 400 V/xcexcm.
In our opinion, the microphone chip designs based on an insulating diaphragm material are to be preferred from a fabrication point-of-view. There are several conducting diaphragm materials that can be made on silicon wafers. In the table below, we show a list of materials, together with the disadvantages.
With most of the conductive diaphragm materials, the stress cannot be controlled, whereas stress is an extremely important parameter for controlling microphone parameters such as sensitivity and resonance frequency. The stress of polycrystalline silicon can be controlled with sufficient accuracy, but the fabrication of microphone diaphragms is complicated, since the thin diaphragms have to be protected during the etching of the silicon wafer.
A very attractive insulating diaphragm material is silicon nitride. The stress of the silicon nitride layers can be accurately controlled, and the fabrication of diaphragms is easy, since silicon nitride is hardly attacked by the silicon etchant. Therefore, we consider silicon nitride to be a better diaphragm material than the available conducting materials.
A problem with the known chip designs in FIG. 3 and FIG. 4 is that the bulk properties of silicon nitride are not good enough at the extremely high electrical field strength when using the microphone chip at 200 V polarization voltage. We have for example measured a leakage resistance of 1 Gxcexa9 at 400 V/xcexcm (200V across a 0.5 xcexcm silicon nitride diaphragm). Increasing the diaphragm thickness is not a solution to this problem, since the diaphragm stiffness then also increases. This increase in stiffness can be compensated by a decrease in diaphragm stress, which is done in practice by changing the composition of the silicon nitride to a more silicon-rich composition. A problem is that the insulating properties of silicon nitride degrade rapidly when shifting to more silicon-rich compositions, so that the advantage of using a higher thickness is gone. Information about this can be found in the Ph.D. thesis xe2x80x9cResonating microbridge mass flow sensorxe2x80x9d, by S. Bouwstra, University of Twente, The Netherlands, March 1990, pp. 52-56. Another way to get around this stiffness problem is to thin down the diaphragm after silicon nitride deposition. This is a critical process that is difficult to do at wafer level in production.
Much of what is stated above in relation to condenser microphones also applies to capacitive electrical components in general and to MEMS components in particular.
A much more simple method is proposed here for improving the leakage resistance of microphone chips, by adding an extra insulator to the design, which ensures that the electrical field strength in the insulator always stays below values where the bulk leakage resistance becomes too low, say  less than 50 V/xcexcm.
Thus a new design is proposed for a micromachined capacitive electrical component such as a condenser microphone, having the following characteristics:
1. A non-conductive diaphragm, preferably from silicon nitride,
2. A high bulk leakage resistance between the diaphragm electrode and the back plate electrode, obtained by adding an extra insulator,
3. A high surface leakage resistance between the diaphragm electrode and the back plate electrode, obtained by designing a large lateral distance between the diaphragm electrode and the back plate electrode, and
4. An air gap thickness larger than 10 xcexcm, securing that a low-stiffness diaphragm can be used in combination with a polarization voltage up to 200 V.